A virtual tape library device that virtually performs a tape operation on a disk and that holds image data (hereinafter referred to as data) on a physical tape contained in a tape library on a disk array as logical volume (LV) has been researched. Such a virtual tape library device does not require mechanical operations, e.g., conventional tape mounting or load/unload, so that high-speed processing can be realized. This virtual tape library device includes a hierarchical control server that controls data or the like between a host and a tape library. When receiving a data read/write request from the host, the virtual tape library device behaves as a tape library and reads/writes data by using a disk array.
As described above, the virtual tape library device responds to the host by using a disk array device having a high data access speed, and thus enables higher-speed processing than a system using only a conventional tape library. After responding to the host, the virtual tape library device stores data written on the disk array device on a tape in the background without communicating with the host. This is called a migration process.
In the virtual tape library device, update of data is not frequently performed and large-volume data that has been migrated is erased from the disk array device in order to prevent the space in the disk array device from being insufficient due to the data written thereon. In a case where data requested by the host is “on cache”, existing on the disk array device, the virtual tape library device reads the data from the disk array device and responds to the host. On the other hand, in a case of “cache miss” where data for a read/write response to the host does not exist on the disk array device, the virtual tape library device reads migrated data that is stored on a tape contained in the tape library. After writing the read data on the disk array device, the virtual tape library device responds to the host by using the data on the disk array device.
According to a related art, a plurality of buffer memories are provided on a transmitter side. For example, Japanese Unexamined Patent Application Publication No. 2006-287293 discloses a technique of generating a pseudo response immediately after data transfer from the buffer memories and enabling storage of other data to be transferred in a case where the buffer memories have a free space.
In a tape library connected to an open-type host, the volume of data written from the host is expanded compared to that in a main-frame type, and thus data that is not frequently updated after a migration process is erased from a disk array. This increases, however, the possibility that a cache-miss state occurs. As described above, the conventional virtual tape library device transfers data to the host after migrating the data to the disk array device. Therefore, conventionally, a processing time is disadvantageously long in a case where large-volume data is to be handled on the host side. Accordingly, in the case where large-volume data is to be handled on the host side, time for responding to the host becomes longer in a system including the virtual tape library compared to a system including only an ordinary tape library.